The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (e-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, an e-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 110a-c, commonly referred to as eNodeB (eNB). The eNBs serve one or more areas referred to as cells 120a-c and may communicate with each other over the X2 interface. In FIG. 1a the UE 150 is served by the cell 120a of the serving eNB 110a. Cells 120b and 120c are neighboring cells.
One of the 3GPP LTE features with higher performance requirements is the handover (HO). The HO performance is of key importance in an LTE system, since the requirements of mobility performance are stringent, namely mobility should be maintained with good performance at speeds of up to 350 km/h. In addition, LTE is designed to guarantee lower latency for both the control plane and the user plane compared to the previous systems, e.g. UMTS, with the use of a dedicated preamble in the random access procedure. Hence, in the case of HO, when interruption in the data and control plane occur, LTE can achieve 12-22 ms user plane interruption in both uplink and downlink in normal cases. In this respect, it is important to devise mechanisms which maintain the good HO performance in terms of interruption time even in challenging scenarios.
The HO procedure may be described as a group of consecutive HO Radio Resource Control (RRC) signaling messages between a UE and a serving or source eNB. Several X2 messages for the signaling between the source and target eNB are also needed when the source and target cells are in different eNBs. The HO procedure is schematically illustrated in FIG. 2. The procedure is started when a UE transmits a measurement report message 201 to the source eNB, where the measurement report indicates that a HO is needed. The measurement report thus triggers the HO procedure, and is based on downlink radio channel quality measurements in serving and neighboring cells. The measurement report is transmitted if the radio quality of the serving cell is typically a number of dB lower than that of a neighboring or target cell (corresponding to the so called HO Hysteresis Trigger) for a defined period (corresponding to the so called HO Time-to-Trigger). The UE reports a “best cell” list including all the neighboring cells which has higher radio channel quality than the serving cell. The source eNB makes a HO decision 202 based on the “best cell” list in the measurement report, and sends a HO request 203 via the X2 interface to the eNB that is determined to be the target. Such a HO request 203 is only admitted if the cell of the target eNB has available resources. If there are available resources, the target eNB reserves the resources 204, and sends back a HO request acknowledgement 205 to the source eNB. When the source eNB receives the acknowledgement 205, it sends a HO command 206 to the UE, which will then switch 207 to the target cell. From a UE perspective, the HO procedure ends when the UE has accessed the target cell successfully through the random access procedure 208 and therefore transmits a HO complete message 209 to the target eNB. The target eNB will also ask the source eNB to release the UE resources 210.
Radio link failure (RLF) may be detected if the radio channel quality is bad. Within 3GPP LTE, this occurs upon detection of N out-of-sync indications from the physical layer, or when the maximum number of radio link control (RLC) retransmissions is reached. RLC is the link-layer protocol responsible for error recovery and flow control in UTRAN and e-UTRAN. RRC protocol data units are normally transmitted in RLC Acknowledge Mode (AM). If the RLC transmitter can not get an acknowledgement within a predefined period, which corresponds to the maximum number of RLC retransmissions, the corresponding RLC connection may be reset and re-established. These are the cases when RLF is detected in general. These situations are also the reasons for RLF during handover. Moreover, in the case of handover, RLF can be detected when the maximum number of random access attempts in the target cell has been reached.
A RLF recovery procedure is illustrated in FIG. 3a. Two phases 301 and 302 characterize the procedure. The first phase 301 is started upon the start of the radio problem detection 303 for a UE in connected mode 304. The radio problem detection 303 may e.g. be the detection of N out-of-sync indications from the physical layer as described above. A timer T1 defining the first phase 301 is started at the detection of the first out-of-synch indication, and if the radio link quality does not recover before the timer T1 expires, the radio link failure detection will be triggered at t0. This detection ends the first phase 301 and starts the second phase 302. When the timer T2 of the second phase expires, the UE will switch to an idle mode 305, unless it has succeeded to recover to a “best cell” and can remain in a connected mode.
The time for a successful RLF recovery procedure is in the range of several hundreds of milliseconds up to a few seconds. FIG. 3b illustrates schematically the time periods needed in order to successfully recover from a detected RLF, and gives a more detailed view of what happens during T2 in the second phase 302 in FIG. 3a. At t0 the RLF is detected, at t1 the UE is synchronized to the system, at t2 the “best cell” selection is made, at t3 random access is started and at t4 the random access succeeds. Finally, at t5 the RRC connection reestablishment is completed, and if this is achieved before T2 expires, then the RLF recovery procedure is successful. The different time periods of the procedure are explained in the following with typical values within parenthesis:                T21: Synchronize (up to 100 ms)        T22: Perform one measurement (66 ms)        T23: Read the BCCH (0-240 ms)        T24: Perform random access procedure (dependent of RACH)        T25: Perform RRC connection reestablishment (typically a few dozens of ms if cell prepared, in the order of 150 ms otherwise)        
In some situations the HO procedure is triggered when radio link quality is degrading, but the UE will still encounter bad radio link conditions. There are numerous reasons for this. One reason may be that the HO Hysteresis Trigger and HO Time-to-Trigger are not set appropriately, and another reason may be that the UE is moving very fast and the link adaptation is thus not working properly. Still another reason is that the UE remains in the same cell, even if the HO triggers are set appropriately. This might happen when the UE has triggered a HO, the serving eNB has decided to HO the UE to a target cell indicated by the UE, but the target cell has not acknowledged the HO request due to lack of available resources.
This scenario is illustrated in FIG. 4. Similarly to what is described above with reference to FIG. 2, a measurement report 401 triggering the HO procedure is transmitted to the source eNB, and the source eNB makes a HO decision 402 and sends a HO request 403 to the target eNB. The reservation of resources is then performed with an admission control (AC) procedure 404 in the target eNB. If the AC indicates that the UE requesting access can not be admitted in the cell, the target eNB sends a non acknowledgement (NACK) of the HO request 405 to the source eNB. As the UE will not receive any HO command in this case, it will stay with the source cell, and continue with the data transmission as usual although the radio channel quality may get worse. After a few hundreds of milliseconds the UE will very likely again transmit a measurement report 406 to the source eNB, still indicating the need for a HO. One option is that, the target cell—which may be another one than in the first case—will have available resources 408 and will acknowledge 409 the HO request 407. If the HO request still returns a NACK due to AC rejection, the UE will have to stay with the source cell for even longer time. The UE in such a scenario may be very close to the source cell edge and will thus very likely have a bad radio link to its source cell. The bad radio conditions will most probably trigger a RLF sooner or later.
FIG. 5 illustrates the probability that a UE will attempt to recover in the serving cell 501, in the target cell 502 or in another third cell 503 in case of a RLF during the HO procedure to a target cell. As already mentioned, a UE will eventually perform the UE-mobility based RLF procedure described above with reference to FIGS. 3a-b. The UE will try to recover in a selected “best cell” in the RLF recovery process. As illustrated in FIG. 5, the probability that a UE will attempt to recover in the target cell is about 70-80%. The probability that the UE will get rejected again during RLF recovery is thus high, as it is not likely that the target cell releases the currently allocated resources and gets enough available resources for the new requests within a few hundred milliseconds. Consequently, this is a problem that results in an increase of the HO delay. Another negative impact is an increase of the HO failure recovery delay, which may result in a lower user satisfaction when it comes to delay-sensitive traffic. Furthermore, if the UE cannot recover within the periods T1+T2 during the RLF procedure (see FIG. 3a) the UE will go to idle mode, which will cause even longer delays due to that a connection reestablishment is needed.